Accurate Fish Weight Without A Scale Often we find ourselves with a big fish and no scale. When this happens some genius came up with a way to get the approximate weight by a simple formula. It works on all heavy bodied deep sea fish and gives you a fairly accurate estimate on the weight. It works as follows. Measure the girth at the fish’s greatest diameter and square it. Then measure the length from the notch of the tail to the tip of the lower jaw. Next, multiply the square of the girth times the length of the fish and divide by 800. The total is your fish’s weight. Try this when you have a chance and you will find it’s amazingly accurate. Example a fish that is 38 inches in girth and 59 inches in length. 38x38=1,444, and 59x1,444=85,196. 85196 divided 800=106.49 0r 106 ½ pounds.